The present invention relates generally to electrical machines and, more particularly, to a synchronous reluctance electrical machine topology that utilizes a rotor retained by a composite sleeve.
High speed, high power density electrical machines are essential to traction motor applications. To get the most power per unit volume, permanent magnet electrical machine are typically used for their many desirable attributes. While interior permanent magnet (IPM) machines have been the primary candidates for traction motors in light-duty hybrid/electric vehicles (HEV/EV), the price and availability of magnets have been a cause of concern. Typically these motors use Neodymium Iron Boron (NdFeB) permanent magnets, which contain both light rare earth materials as Neodymium (Nd) as well as heavy rare-earth materials as Dysprosium (Dy). One of the key risks in terms of using these rare-earth magnets is the significant fluctuation/increase in their prices over the past few years. Traction applications as well as wind generators that use large quantities of these magnets were the most affected by these fluctuations. There has been an ongoing global effort to try to reduce or eliminate the use of rare earth materials without sacrificing performance. As a part of elimination of rare-earth materials, conventional topologies such as induction, switched reluctance and synchronous reluctance are being considered as alternatives.
Synchronous reluctance machines are particularly appealing due to their smooth and simple passive rotor structure, i.e., the absence of magnetic flux exciting structures like magnets or coils), comparable power density to induction motors, low rotor losses, absence of magnets, and simple control. The absence of magnets in this configuration is attractive particularly because of the volatility of rare-earth material prices. Additionally, the risk of demagnetization or uncontrolled back-emf generation (the back-emf of the motor exceeds the DC link voltage) is eliminated. The key disadvantages of the synchronous reluctance machine are low power factor and typically limited constant power speed ratio (CPSR). This is mainly due to the presence of bridges and/or center-posts, especially in high-speed machines. FIG. 1 illustrates a conventional synchronous reluctance machine 10 with bridges 12 and center posts 14. The bridges 12 and center posts 14 provide a path for increasing the leakage flux in the synchronous reluctance machine 10. Since this leakage flux does not add to the electromagnetic torque provided, the loss of flux to leakage reduces the torque and hence the power factor of the machine 10. Additionally, the inductance from the leakage flux adds to the machine voltage limiting the voltage available for torque production. The limitation of the voltage is especially problematic at high speed operation, where the presence of the bridge 12 and center posts 14 reduces voltage available to torque production and leads to a precipitous drop in the output torque, i.e, a lower constant power to speed ratio (CPSR).
High speed synchronous reluctance machines with large rotor diameters require robust physical retention. A conventional synchronous reluctance machine 10 relies on flux paths through the rotor that must be retained by physical features to prevent them from deforming into the air-gap of the machine 10. These retention features are typically built into the configuration of the rotor laminations in the form of bridges 12 and center posts 14. The higher the speed and larger the rotor diameter gets, the thicker the bridges 12 and center posts 14 need to be. The thicker these features are, the lower the performance capability of the machine 10. The performance of the conventional synchronous reluctance machine 10 is typically not as good as permanent magnet versions, especially in high speed applications because of the mechanical limitations of the spinning rotor structure.
Thus, there is a need for a synchronous reluctance electrical machine that mitigates the need for center posts as well as being suitable for high-speed operation. Reducing the center posts would significantly ease the issue of leakage paths in the center posts as well as reduce torque ripple and rotor losses.